Earthquake

EarthquakeEarthquake

• Foci – location inside earth where an Foci – location inside earth where an earthquake startsearthquake starts

• Epicenter – Point on earth’s surface Epicenter – Point on earth’s surface directly above the focusdirectly above the focus

Anatomy of EarthquakesAnatomy of Earthquakes

• Amount of Damage is dependant on Amount of Damage is dependant on depthdepth

• Foci is close to surface – lots of Foci is close to surface – lots of damagedamage

• Foci is far from surface – little Foci is far from surface – little damage – energy has to travel a long damage – energy has to travel a long wayway

Seismic waves – p 529Seismic waves – p 529

• 3 Types: P, S, and L3 Types: P, S, and L

• L waves – surface waves – roll along L waves – surface waves – roll along the earth’s surfacethe earth’s surface

• Body waves: P and S wavesBody waves: P and S waves

• Body waves travel faster than Body waves travel faster than surface waves- feel p and s waves surface waves- feel p and s waves first after an earthquakefirst after an earthquake

Body wavesBody waves

• P wave- Primary wave -first to arriveP wave- Primary wave -first to arrive– Ground squeezes and streches in the Ground squeezes and streches in the

direction of wave traveldirection of wave travel– Travels through solids and liquidsTravels through solids and liquids

• S wave- secondary wave -second to S wave- secondary wave -second to arrivearrive– Ground is perpendicular (90) to direction of Ground is perpendicular (90) to direction of

wave travelwave travel– Travels through solids but not liquidsTravels through solids but not liquids– Another name for S-wave is shear waveAnother name for S-wave is shear wave

Smaller amplitude than surface (L) waves, but faster, P arrives first

P and S waves

Two Types of Surface Two Types of Surface WavesWaves

Most of the destructionLarger amplitude than body waves

Why do P waves reach stations Why do P waves reach stations before S?before S?

• Outer core is Liquid – S waves can’t Outer core is Liquid – S waves can’t travel through, but p can p 530travel through, but p can p 530

Surface wavesSurface waves

• L wave – Travels along the earth’s L wave – Travels along the earth’s surfacesurface

• Last to arriveLast to arrive

• Ground motion is a rolling action like Ground motion is a rolling action like ripples on a pondripples on a pond

SeismologySeismology

SeismometersSeismometers - instruments that - instruments that record seismic wavesrecord seismic waves

•Records the movement of Records the movement of Earth in relation to a stationary Earth in relation to a stationary mass on a rotating drum or mass on a rotating drum or magnetic tapemagnetic tape

A seismograph designed to A seismograph designed to record vertical ground record vertical ground motionmotion

The heavy mass doesn’t move muchThe drum moves

Lateral Movement DetectorLateral Movement Detector

In reality, copper wire coils move around magnets, generating current which is recorded.

• Seismograms tell :Seismograms tell :

• How long an earthquake lasted and How long an earthquake lasted and the amount of ground shakingthe amount of ground shaking

• Magnitude – rates the strength of an Magnitude – rates the strength of an earthquakeearthquake

• Higher the magnitude the stronger Higher the magnitude the stronger the earthquakethe earthquake

• Each 1 step increase in magnitude = Each 1 step increase in magnitude = 10 times the size of seismic waves10 times the size of seismic waves

• Seismograms determine how fast Seismograms determine how fast seismic waves are travelling.seismic waves are travelling.

• Look at p531Look at p531

• P waves = 5.5 min and S = 10 minP waves = 5.5 min and S = 10 min

Earthquake focus and Earthquake focus and epicenterepicenter

Note how much bigger the surface waves are

Body Waves

Delay between P and S arrivals gives distance to epicenter

Graph to find distance to Graph to find distance to epicenter epicenter

• Don’t need to know the next few Don’t need to know the next few slides, but it is cool to do ;)slides, but it is cool to do ;)

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

• Three seismographs needed to locate an Three seismographs needed to locate an epicenterepicenter

• Each station determines the time interval Each station determines the time interval between the arrival of the first P wave and between the arrival of the first P wave and the first S wave at their locationthe first S wave at their location

• A travel-time graph then determines each A travel-time graph then determines each station’s distance to the epicenterstation’s distance to the epicenter

Locating Earthquake Epicenter Locating Earthquake Epicenter

• Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

•A circle with radius equal to distance to A circle with radius equal to distance to the epicenter is drawn around each the epicenter is drawn around each stationstation

•The point where all three circles The point where all three circles intersect is the earthquake epicenterintersect is the earthquake epicenter

Epicenter located using three Epicenter located using three seismographsseismographs

Earthquake BeltsEarthquake Belts

95% of energy released by earthquakes 95% of energy released by earthquakes originates in narrow zones that wind originates in narrow zones that wind around the Eartharound the Earth

•These zones mark of edges of These zones mark of edges of tectonic platestectonic plates

Locations of earthquakesLocations of earthquakesfrom 1980 to 1990from 1980 to 1990

Broad bands are subduction zone earthquakes, narrow are MOR

80% of seismic energy around Pacific Rim